Synthesis of Highly Crystalline NH2-MIL-125 (Ti) with S-Shaped Water

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Synthesis of Highly Crystalline NH-MIL-125 (Ti) with SShaped Water Isotherms for Adsorption Heat Transformation Muhammad Sohail, Yang-No Yun, Eunkyung Lee, Sang Kyum Kim, Kanghee Cho, Jong-Nam Kim, Tae Woo Kim, Jong-Ho Moon, and Hyunuk Kim Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01597 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 24, 2017

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Crystal Growth & Design

Synthesis of Highly Crystalline NH2-MIL-125 (Ti) with S-Shaped Water Isotherms for Adsorption Heat Transformation Muhammad Sohail,†,‡ Yang-No Yun,† Eunkyung Lee,§ Sang Kyum Kim,§ Kanghee Cho,§ Jong-Nam Kim,§ Tae Woo Kim,† Jong-Ho Moon,ǁ Hyunuk Kim*,†,‡ †

Energy Materials Laboratory and §Green Fuel Laboratory and ǁGreenhouse Gas Laboratory, Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea

‡

Advanced Energy Technology, University of Science and Technology, 217 Gajeongro, Yuseong-gu, Daejeon 34113, Republic of Korea

ABSTRACT: NH2-MIL-125 has been investigated as a water adsorbent because of its high hydrothermal stability and S-shaped water adsorption isotherm. Herein, we report the synthesis of NH2-MIL-125 with high surface area and water capacity for an Adsorption Heat Transformation (AHT) system. NH2-MIL-125 derived from Ti(BuO)4 shows higher surface area and water uptake than those of Ti(iPrO)4-derived samples regardless of the synthesis method. In a sense of crystallinity, a solvothermal method with static conditions generated more distinct crystalline properties than the one synthesized by a reflux reaction as confirmed from PXRD analysis, UV–vis absorbance spectra and SEM images. Considering it as an adsorbent for AHT system, Ti(BuO)4-derived sample synthesized by a solvothermal method shows an ideal Sshaped isotherm with a steep rise of water uptake at lower relative pressure (0.550 g/g at P/P0 = 0.30), which is attributed to narrow triangle apertures and hydrophilic functional groups. This material shows the dynamic water adsorption/desorption cycle without any noticeable weight change.

*Corresponding authors; Name: Hyunuk Kim; tel/fax: +82 42 860 3531; email: [email protected] 1 ACS Paragon Plus Environment


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Synthesis of Highly Crystalline NH2-MIL-125 (Ti) with S-Shaped Water Isotherms for Adsorption Heat Transformation Muhammad Sohail,†,‡ Yang-No Yun,† Eunkyung Lee,§ Sang Kyum Kim,§ Kanghee Cho,§ JongNam Kim,§ Tae Woo Kim,† Jong-Ho Moon,ǁ Hyunuk Kim*,†,‡ †

Energy Materials Laboratory and §Green Fuel Laboratory and ǁGreenhouse Gas Laboratory,

Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon, 34129, Republic of Korea ‡

Advanced Energy Technology, University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea

KEYWORDS: Metal-Organic Framework, Water Adsorption, Adsorption Heat Transformation

ABSTRACT: NH2-MIL-125 has been investigated as a water adsorbent because of its high hydrothermal stability and S-shaped water adsorption isotherm. Herein, we report the synthesis of NH2-MIL-125 with high surface area and water capacity for an Adsorption Heat Transformation (AHT) system. NH2-MIL-125 derived from Ti(BuO)4 shows higher surface area and water uptake than those of Ti(iPrO)4-derived samples regardless of the synthesis method. In a sense of crystallinity, a solvothermal method with static conditions generated more distinct

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crystalline properties than the one synthesized by a reflux reaction as confirmed from PXRD analysis, UV–vis absorbance spectra and SEM images. Considering it as an adsorbent for AHT system, Ti(BuO)4-derived sample synthesized by a solvothermal method shows an ideal Sshaped isotherm with a steep rise of water uptake at lower relative pressure (0.550 g/g at P/P0 = 0.30), which is attributed to narrow triangle apertures and hydrophilic functional groups. This material shows the dynamic water adsorption/desorption cycle without any noticeable weight change.

INTRODUCTION Global Energy consumption in buildings accounts for 20 to 40 % of the total energy in developed countries.1 Air conditioning of buildings operated by electrical energy utilizes a prominent part of this energy. The rise in demand for electrical energy, therefore, encourages researchers to investigate energy-efficient systems for conditioning buildings based on alternative energy sources such as low temperature industrial waste heat or solar energy. Adsorption Heat Transformation (AHT) system, using heat released and absorbed by physical adsorption and desorption of guest molecules, is considered as a viable alternative to traditional air conditioning systems. In an air conditioning system based on AHT, evaporation of the working fluid for cold generation, its adsorption by porous materials and regeneration of adsorbent by low temperature heat, are the three steps of working cycle for its operation. Among various working fluids, water is preferred over other fluids because of its high evaporation enthalpy (2440 kJ/kg at 25 °C) and non-toxicity.2 In AHT systems, the high water uptake and lower regeneration temperature of adsorbents at working conditions are critically important to improve the working efficiency of air conditioning. Considering working conditions of an AHT system driven by low temperature heat and low humidity, therefore, porous materials should


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have a steep increase of water uptake in the relative humidity range of 0.05 < P/P0 < 0.32 representing an S-shaped isotherm curve.3 Until now, zeolites and silica gels have been commercially used for AHT systems, but there are still several challenging issues.4,5 For instance, since zeolites have low surface area and strong affinity to water, they have relatively low water uptake and high regeneration temperature.3,6 In the case of silica gels, they have low water capacity at low pressure because of less hydrophilic nature.3,6 These limitations motivated researchers to search novel adsorbents. Metal-Organic Frameworks (MOFs) are the emerging adsorbents with well-ordered network structures assembled by coordination bonds between metal ions and organic ligands. MOFs with various topologies have been synthesized and investigated for applications in gas adsorption and separation,7-11 catalysis,12, 13 energy storage,14 electrochemistry,15 magnetism,16 drug delivery,17 and several other fields.18-20 Among these applications, the AHT system operated by MOFs has not been developed because of its low water stability and high production cost, even though they have high water uptake based on large surface area and functionality.21,22 The low stability of MOFs in the presence of water is due to the dissociation of weak coordination bonds between metal and organic ligands. Novel water-stable MOFs with oxophilic metals such as Zr (IV) and Ti (IV), however, have been recently reported in literature.23-27 NH2-MIL-125 has been investigated as a potential adsorbent for AHT systems because of its high water stability and capacity at operating conditions.27 This framework with Ti8O8(OH)4(O2CC6H5-CO2-NH2)6 as a basic unit, has a bipyramid structure with six cyclic octamers Ti8O20(OH)4 at the corner connected by eight NH2-BDC linkers (Figure 1 and S1).28 Herein, we report the synthesis of NH2-MIL-125(Ti) with high surface area and microporosity by controlling Ti(IV) precursors and synthetic methods. Interestingly, this porous material shows an ideal S-


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shaped isotherm with a steep increase of water uptake at lower relative pressure and adsorption temperature of 35 °C. Dynamic water sorption experiments confirmed that NH2-MIL-125(Ti) maintains high stability without any notable decrease of water uptake during adsorptiondesorption cycles. For a practical application, NH2-MIL-125 crystals were synthesized in bulk quantity and were pelletized to achieve high density of the adsorbent.

Figure 1. (a) The crystal structure of NH2-MIL-125 (Ti) (b) the simplified skeleton of the framework with a polyhedral drawing (TiO6) RESULTS AND DISCUSSION Characterization of NH2-MIL-125 Synthesized by Reflux Method. The polycrystalline products of NH2-MIL-125 were synthesized from two different Ti(IV) precursors such as Ti(iPrO)4 and Ti(BuO)4 under a reflux reaction. The crystal phases of samples were investigated by powder X-ray diffraction (PXRD) analysis. As shown in Figure 2a, PXRD profiles of NH2MIL-125 well match with the simulated pattern generated from the reported crystal structure, which confirms that both porous materials have high phase purity.28 The close inspection of PXRD profiles revealed that the normalized main (101) peaks of two samples have different peak broadening related to the crystallinity (Figure S2). The Ti(BuO)4-derived sample has more


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narrow peak width than that of the Ti(iPrO)4-derived one indicating that NH2-MIL-125 synthesized from Ti(BuO)4 has a higher crystallinity than that from Ti(iPrO)4. Mean crystal domain sizes estimated by Scherrer’s equation are ca. 46 nm for the Ti(BuO)4-derived sample and ca. 27 nm for the Ti(iPrO)4-derived sample. As shown in SEM images (Figure 2b), the Ti(BuO)4-derived sample has regular square plate shape with hundreds of nm size, whereas the Ti(iPrO)4-derived sample has irregular shape. This indicates that the crystal grain of the Ti(BuO)4-derived sample grew well more than that of the Ti(iPrO)4-derived one, which is consistent with results observed from PXRD profiles. To gain insight of characteristics for surface area and porosity, nitrogen adsorption experiments were carried out at 77 K (Figure 3). The BET surface area (ca. 1509 m2/g) of Ti(BuO)4-derived sample is slightly higher than that (ca. 1358 m2/g) of Ti(iPrO)4-derived sample. The difference in the BET surface area is mostly attributed to micropore volumes of porous materials (Table 1).

Figure 2. (a) Powder XRD profiles and (b) SEM images of NH2-MIL-125 synthesized using Ti(BuO)4 and Ti(iPrO)4 under a reflux method.


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Figure 3. N2 adsorption isotherms of NH2-MIL-125 synthesized by a reflux method (squares: Ti(BuO)4, circles: Ti(iPrO)4).

Table 1. Gas adsorption properties of NH2-MIL-125

Synthetic method

Synthetic method

BET surface area (m2/g)

Total pore volume (cm3/g) at P/P0=0.99

Micropore volume a (cm3/g)

Mesopore volume b (cm3/g)

Ti(BuO)4

1,509

0.6622

0.5969

0.0653

Ti(iPrO)4

1,358

0.5803

0.5443

0.0360

Ti(BuO)4

1,553

0.6359

0.6077

0.0282

Ti(iPrO)4

1,340

0.5426

0.5312

0.0114

Reflux

Solvothermal a

Determined by t-plot analysis.

b

Mesopore volume = Total pore volume – micropore volume


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Characterization of NH2-MIL-125 Synthesized by Solvothermal Method. To achieve high crystallinity, NH2-MIL-125 crystals were synthesized by a solvothermal reaction in a teflon-lined autoclave. PXRD analysis revealed that NH2-MIL-125 samples synthesized from Ti(BuO)4 and Ti(iPrO)4 have high crystallinity and phase purity as their PXRD peaks matches well with the simulated pattern (Figure 4a). The Scherrer’s equation provides that Ti(BuO)4 and Ti(iPrO)4 have ca. 58 and 43 nm of crystal grain size, respectively (Figure S3). Compared to crystal grain sizes of NH2-MIL-125 synthesized by a reflux method, the solvothermal method with static crystallization process generates bigger crystal grain sizes. SEM images also revealed that NH2MIL-125 synthesized by a solvothermal method has more distinct crystalline morphology with a shape of truncated bipyramid than that by a reflux method.18 The specific surface area and micropore volume determined by nitrogen adsorption at 77 K are 1,553 m2/g and 0.6077 cm3/g for Ti(BuO)4-derived NH2-MIL-125, and 1,340 m2/g and 0.5312 cm3/g for Ti(iPrO)4-derived NH2-MIL-125 (Table 1). These values are quite comparable to those synthesized by a reflux method even though these materials have higher crystallinity. NH2-MIL-125 synthesized by using Ti(BuO)4 has higher surface area and crystallinity than the ones using Ti(iPrO)4. At the moment, the possibility that alkoxy groups in titanium precursors play an important role during the crystal growth and pore formation of NH2-MIL-125 cannot be excluded. For instance, it is reported that the alkoxy groups evidently influence the kinetics of hydrolysis and condensation reactions in sol-gel reactions of titanium alkoxides.19 In this study, the effect of the alkoxy groups on the crystal growth and the pore formation is still not clear. However, it is the fact that the samples synthesized from Ti(BuO)4 possess a higher crystallinity and regular pore structure than those from Ti(iPrO)4, which affect the diffusion of N2 gas into the pores to achieve high surface area and pore volume. Therefore, Ti(BuO)4 is a


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more suitable precursor than Ti(iPrO)4 to synthesize NH2-MIL-125 regardless of the synthesis methods.

Figure 4. (a) Powder XRD profiles and (b) SEM images of NH2-MIL-125(Ti) synthesized using Ti(BuO)4 and Ti(iPrO)4 by a solvothermal method.


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Figure 5. N2 adsorption isotherms of NH2-MIL-125(Ti) synthesized by a solvothermal method (squares: Ti(BuO)4, circles: Ti(iPrO)4) Optical Properties of NH2-MIL-125. As mentioned above, a solvothermal method under static conditions generated higher crystallinity than that obtained by a reflux reaction. Interestingly, NH2-MIL-125 synthesized from Ti(BuO)4 shows different UV-vis spectra depending on synthetic methods (Figure S4). HOMO (highest occupied molecular orbital)– LUMO (lowest occupied molecular orbital) band gaps of NH2-MIL-125 estimated with KubelkaMunk function from UV–vis absorbance spectra are 2.71 eV for a solvothermal method and 2.75 eV for a reflux method (Figure 6). NH2-MIL-125 synthesized by a solvothermal reaction has slightly narrow band gap than that by a reflux method. In general, the band structure in the semiconducting materials is affected by various factors such as crystal size, morphology, electronic structure, defects (ex. vacancies), etc.29 In this respect, the more narrow band gap for


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NH2-MIL-125 synthesized by a solvothermal method may be attributed to the higher crystallinity, which results in the well-organized orbital interactions through the strong chemical bonding between Ti(IV) and 2-amino terephthalate. As can be seen in SEM images (Figure 2b and 4b), indeed, solvothermally synthesized NH2-MIL-125 has bigger crystal grain size and more clear crystal shape than that by a reflux method.

Figure 6. Diffuse reflectance UV-vis absorption spectra of NH2-MIL-125based on KubelkaMunk function vs. the photon energy (blue line; solvothermal method, red line; reflux method). Water Adsorption Isotherms of NH2-MIL-125: The water adsorption and desorption isotherm of Ti(BuO)4-derived NH2-MIL-125 was volumetrically measured on relative pressure. NH2-MIL-125 shows an S-shaped isotherm with steep rise in the range of 0.1 < P/P0 < 0.3,


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which is desirable in adsorbent based low heat transformation systems. In addition, the isotherm shows almost no hysteresis, which is also useful for practical applications (Figure 7). The water uptakes of NH2-MIL-125 at 35 °C are 0.550 and 0.670 g/g at P/P0 = 0.30 and 0.92, respectively. The Ti(iPrO)4-derived NH2-MIL-125, however, showed much lower water uptake (0.370 and 0.440 g/g at P/P0 = 0.30 and 0.92, respectively) than those of Ti(BuO)4-derived sample, even though it has a similar S-shaped isotherm (Figure S5). Higher water uptake of the Ti(BuO)4derived sample is attributed to its high surface area, pore volume and crystallinity as discussed above. The S-shape of an isotherm can be explained by the pore structure with a narrow neck and large internal void space. The aperture with a narrow neck prevents the fast diffusion of water at the early stage, and later allows a steep water uptake in large void space above a certain pressure. To estimate the pore accessibility by water, the window sizes of NH2-MIL-125 were determined by the modeling program. There can be three types of apertures because of the disorder problem of amino groups along the a-axis. The aperture sizes with two amino groups, one amino group and no amino group in triangle windows are 3.4, 3.8 and 4.2 Å, respectively, which are slightly larger than the kinetic diameter (2.6 Å) of water. The space filling model of NH2-MIL-125, however, represents that the water cannot penetrate through a cyclic octamer, Ti8O20(OH)4 along c axis because of the small window size (2.4 Å) considering van der Waals radius (Figure S6). For comparison, water uptake of MIL-125 with same topology was measured at the same condition. The water isotherm of MIL-125 also shows a similar S-shaped curve because of the pore structure with narrow neck and large void space (Figure S7). Therefore, we consider that this type of pore structure is critically important to have an S-shaped curve.


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To affect the starting point of water uptake for isotherm showing an S-shaped curve, the hydrophilicity of porous materials plays an important role. For example, the steep water uptake of NH2-MIL-125 occurs at lower pressure than that of MIL-125 because of hydrophilic functional groups (Figure S7). The increase of hydrophilicity by amine group was also noted for NH2-UiO-6730 and MIL-101Cr.

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With considering the working pressure for an AHT system,

therefore, NH2-MIL-125 is more desirable than MIL-125. The water uptakes of adsorbents with S-shaped isotherms in the range of 0.1 < P/P0 < 0.3 are listed in Table 2. Among many adsorbents, Ti(BuO)4-derived NH2-MIL-125 shows the highest water capacity at P/P0 = 0.3. Table 2. Water adsorption properties of adsorbents Materials

a

Silica SAPO-34 AlPO-18 Ti(BuO)4-derived NH2-MIL-125 Ti(iPrO)4-derived NH2-MIL-125 CAU-10-H A520 MIL-100(Fe) MIL-100(Cr) MIL-101(Cr) NH2-MIL-101(Cr)

BET surface area (m2/g) 838 568 604 1,553

αa

Water uptake (g/g) at P/P0 = 0.3

Maximum water uptake (g/g)

0.22d 0.165e 0.25e

0.33 0.265 0.240

0.44 0.295 0.323

0.23d

0.550

0.683

1,340

0.24d

0.370

0.446

525 1,021 1,917 1,330 3,070 2,890

c

0.29 0.325 0.4 0.11 0.11 0.11

0.35 0.45 0.75 0.40 1.29 0.81

0.18 0.27c 0.29 c 0.36b 0.45c 0.37c

Ref. 32 33 33 This work This work 34 35 24 36 37 37

Relative pressure for which q = 0.5qmax.38 Measured at b293 K, c298 K, d308 K and e313.15K.


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Figure 7. Water sorption isotherm of NH2-MIL-125 at 35 °C, adsorption (●), desorption (○) (α= 0.23).

Figure 8. Pore accessibility of NH2-MIL-125 through triangle windows with (a) two amino groups, (b) one amino group and (c) no amino groups. (Green arrows indicate the neck of a triangle pore).


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To confirm dynamic water uptake and hydrothermal stability of Ti(BuO)4-derived NH2-MIL125 synthesized by a solvothermal method, gravimetric water adsorption/desorption experiments were carried out at working condition of adsorption chiller (Tads = 30 °C, Tdes = 80 °C). As can be seen from Figure 9, the water uptake of sample maintained over 10 cycles without any notable decrease. There is a slight increase in water capacity during the first cycle (from 0.456 g/g to 0.464 g/g), which may be due to further pores cleaning/washing as discussed for the improved crystallinity of MIL-100(Al) after cycling.24 The average water uptake is 0.452 g/g, which is slightly lower than that measured by a volumetric method because of the difference of temperature and relative pressure. PXRD analysis revealed that NH2-MIL-125 maintains the crystallinity after 10 water adsorption/desorption cycles (Figure S8).

Figure 9. (a) Dynamic water adsorption at 30 °C and desorption at 80 °C for NH2-MIL-125, (b) water uptake at 30 °C on repeated cycles. To achieve the high density and avoid the pressure drop, the pellet of NH2-MIL-125 was prepared by mixing with 1 % CMC as a binder (Figure S9). The BET surface area and micropore volume of the pellet are 955 m2/g and 0.3775, respectively (Table S1 and Figure S10). These


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values are lower than those of the as-synthesized sample, but the density of pellet was slightly higher (0.30 g/ml) than that (0.27 g/ml) of the powder material. The water uptake of pellets was 0.400 g/g at P/P0=0.3 (Figure S11).

EXPERIMENTAL SECTION All chemicals and solvents were used as supplied without any further purification. PXRD profiles were recorded on a Rigaku D/max 2500PC diffractometer equipped with a Cu sealed tube (λ=1.54178 Å) at a scan rate of 0.5 °min-1. Scanning electron microscope (SEM) images were acquired on a HITACHI S-4800 SEM. Diffuse Reflectance UV-vis spectra were measured on a SolidSpec-3700 instrument (Shimadzu). The nitrogen and water adsorption isotherms were volumetrically measured by BELSORP-MAX and BELSORP-AQUA3 instruments, respectively (Bel Japan). Dynamic water sorption was gravimetrically measured by a DVS Vacuum instrument (Surface Measurement Systems). The pore accessibility of NH2-MIL-125 was simulated by the Materials Studio program (Acceleys). Synthesis of NH2-MIL-125 by Reflux Reaction. NH2-BDC (3.85 g, 21.2 mmol) was dissolved in anhydrous DMF (50 ml) by stirring it in a 500 ml round bottom flask at 110 °C for 1 hour. Then, anhydrous MeOH (14 ml) was added to the solution and stirred for 1 hour under a reflux condition. After that, Ti(BuO)4 (4.2 ml, 12.3 mmol) was added into the new solution and stirred for 3 days under a reflux condition. After cooling to room temperature, the yellow crystalline product was collected by filtration. This product was washed with DMF to remove unreacted organic ligands, and then DMF was replaced with MeOH by washing twice. After drying under vacuum condition, the obtained product was 4.34 g. Ti(iPrO)4-derived NH2-MIL-


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125 was also prepared by employing the same method with the exception of using Ti(iPrO)4 as a Ti(IV) precursor. The obtained product was 4.47 g Synthesis of NH2-MIL-125 by Solvothermal Reaction. NH2-BDC (1.086 g, 6 mmol) was completely dissolved in a solution of anhydrous DMF (3.5 ml) and MeOH (3.5 ml) by sonication. Ti(BuO)4 (0.51 ml, 1.5 mmol) was added to the solution and sonicated for 5 mins. The solution was then transferred to a Teflon-lined bomb reactor and kept in an oven at 150 °C for 16 hours. After cooling to room temperature, the yellow product was collected by filtration. The product was washed with DMF to remove unreacted organic ligands, and then DMF was replaced with MeOH by washing twice. After drying under vacuum condition, the obtained product was 0.29 g. Ti(iPrO)4-derived NH2-MIL-125 was also prepared by employing the same method with the exception of using Ti(iPrO)4 as a a Ti(IV) precursor. The obtained product was 0.31 g. Scale-up Synthesis of NH2-MIL-125 and Pelletization. The procedure is same as an abovementioned solvothermal method except the amount of reactants. Ti(BuO)4(7.42 ml, 22 mmol), NH2-BDC (16.29 g, 89.9 mmol), DMF (105 ml) and MeOH (105 ml) were used for scale-up synthesis. The obtained product was 6.80 g. To pelletize NH2-MIL-125, 4.5 g of the product was mixed with carboxymethyl cellulose (CMC, 0.045 g, 1.0 %) as a binder in the presence of water (3.5 ml). And, the paste was extruded to prepare pellets and dried under room temperature for 24 hours (Figure S7). Gas and Water Adsorption of NH2-MIL-125. NH2-MIL-125 synthesized by a solvothermal method were activated at 250 °C for 1 day prior to gas and water adsorption measurement. Water adsorption isotherms were volumetrically measured at 35 °C, which is the adsorption temperature for an air conditioning system.5 Nitrogen adsorption isotherms were obtained at 77


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K to determine BET surface area, micropore and total pore volumes. The total pore volume was calculated from the amount of N2 adsorbed at P/P0 = 0.99, by assuming that the pores are then filled with liquid N2. The micropore volume is estimated by t-plot analysis. Dynamic water sorption was gravimetrically measured with 10 cycles. Adsorption and desorption for one cycle maintained for 3 hours each. Water adsorption and desorption were performed at 30 °C and 80 °C, respectively. The relative pressure (P/P0) for water adsorption was 0.289 (12.2 torr).

CONCLUSIONS Crystalline products of NH2-MIL-125 were synthesized from Ti(iPrO)4 and Ti(BuO)4 using reflux and solvothermal reactions. BET, SEM and PXRD analysis revealed that Ti(BuO)4derived NH2-MIL-125 has higher crystallinity and surface area than that synthesized from Ti(iPrO)4 regardless of synthesis methods. Higher crystallinity and surface area of Ti(BuO)4derived NH2-MIL-125 can be attributed to the difference in hydrolysis kinetics of alkoxy groups from titanium alkoxides during the crystal growth. Regarding synthesis methods, a solvothermal method under static conditions generated more distinct crystalline morphology than the one prepared by a reflux reaction. The band gaps of NH2-MIL-125 estimated with Kubelka-Munk function from UV–vis absorbance spectra are well consistent with their crystallinity. Ti(BuO)4-derived NH2-MIL-125 synthesized by a solovthermal method shows an ideal S shaped water adsorption isotherm with a water uptake of 0.550 and 0.670 g/g at P/P0 = 0.30 and 0.92, respectively. The steep rise of water uptake at lower relative humidity is attributed to narrow triangle apertures and hydrophilic functional groups. Dynamic water sorption of NH2-MIL-125 confirms that this sample is stable in the presence of water without any notable weight change. NH2-MIL-125 repeatedly adsorbed 0.450 g/g of water at


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working conditions of the AHT system. Due to an ideal S-shaped isotherm curve and reasonable water stability, Ti(BuO)4-derived NH2-MIL-125 seems to be a potential water adsorbent for the AHT system.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxx. Materials; synthesis of MIL-125; polyhedral and space filling model of NH2-MIL-125; normalized PXRD; diffuse reflectance UV-vis absorption spectra; gas and water adsorption isotherms (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS


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We gratefully acknowledge the Principal Project of the Korea Institute of Energy Research (KIER), which is funded by National Research Council of Science & Technology, Ministry of Science, ICT and Future Planning (MSIP) of the Republic of Korea (B6-2403). We also thank Tae Woo Kim who interpreted PXRD profiles and UV-vis spectra (NRF Basic Science Research Program, 2016R1C1B2010838). REFERENCES (1) Pérez-Lombard, L.; Ortiz, J.; Pout, C. Energy Build. 2008, 40, 394-398. (2) Janiak, C.; Henninger, S. K. Chimia 2013, 67, 419-424. (3) Henninger, S. K.; Jeremias, F.; Kummer, H.; Janiak, C. Eur. J. Inorg. Chem. 2012, 26252634. (4) Aristov, Y. I. Int. J. Refrig. 2009, 32, 675-686. (5) Aristov, Y. Appl. Therm. Eng. 2014, 72, 166-175. (6) Henninger, S. K.; Jeremias, F.; Kummer, H.; Schossig, P.; Henning, H.-M. Energy Procedia. 2012, 30, 279-288. (7) Morris, R. E.; Wheatley, P. S. Angew. Chem. Int. Ed. 2008, 47, 4966-4981. (8) Li, J.-R.; Ma, Y.; McCarthy, M. C.; Sculley, J.; Yu, J.; Jeong, H.-K.; Balbuena, P. B.; Zhou, H.-C. Coord. Chem. Rev. 2011, 255, 1791-1823. (9) Dincă, M.; Long, J. R. Angew. Chem. Int. Ed. 2008, 47, 6766-6779. (10) Li, B.; Wang, H.; Chen, B. Chem. Asian J. 2014, 9, 1474-1498.


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For Table of Contents Use Only

Synthesis of Highly Crystalline NH2-MIL-125 (Ti) with S-Shaped Water Isotherms for Adsorption Heat Transformation Muhammad Sohail, Yang-No Yun, Eunkyung Lee, Sang Kyum Kim, Kanghee Cho, Jong-Nam Kim, Tae Woo Kim, Jong-Ho Moon, Hyunuk Kim*

Highly crystalline NH2-MIL-125 shows an ideal ‘’S’’ shaped water adsorption isotherm with high water capacity for an Adsorbent Heat Transformation (AHT) system. This material shows the dynamic water adsorption/desorption cycle without any noticeable weight change.


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Synthesis of Highly Crystalline NH2-MIL-125 (Ti) with S-Shaped Water

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